1. Field of the Invention
The present invention relates to a method for manufacturing a metal powder suitable for use in electronic components and the like, and relates more particularly to a method for manufacturing a fine, highly crystalline nickel powder of a uniform particle size which is useful as a conductive powder for the conductor pastes used in electronics components.
2. Description of the Related Art
The conductive metal powders used in conductor pastes for forming electronic circuits are desired to be fine powders having few impurities and an average particle size of about 0.01 to 10 μm, and to be composed of monodispersed particles of a uniform size and shape without aggregation. They also need to have good dispersibility in paste, and to have good crystallinity so as not to cause nonuniform sintering.
In particular, when used to form an internal conductor or external conductor in a multilayer capacitor, multilayer inductor or other multilayer ceramic electronic components, a powder needs to have a fine particle size as well as a uniform particle size and shape so that the conductor can be formed as a thin film, and in addition it needs to have a high sintering initiation temperature and be resistant to expansion and contraction caused by oxidation and reduction during sintering so as to prevent delamination, cracks and other structural defects. Consequently, there is demand for submicron-sized nickel powders that are spherical, of low reactivity and highly crystalline.
Conventional methods of manufacturing such highly crystalline nickel powders include a vapor phase chemical reduction method in which nickel chloride vapor is reduced with a reducing gas at a high temperature (see for example Japanese Patent Publication No. 4-365806A), and a spray pyrolysis method in which a solution or suspension of a metal compound dissolved or suspended in water or an organic solvent is formed into fine droplets, and these droplets are heated and thermally decomposed at a high temperature preferably near or above the melting point of the metal to thereby precipitate a metal powder (see for example Japanese Patent Publication No. 62-1807A). A method is also known of thermally decomposing a solid metal compound powder that has been dispersed at a low concentration in a gas phase (see for example Japanese Patent Publication Nos. 2002-20809A & 2004-99992A). In this method, a powder of a thermally decomposable metal compound is supplied using a carrier gas to a reaction vessel where it is dispersed at a low concentration in a gas phase, and then heated at a temperature higher than the decomposition temperature and at or above a temperature (Tm −200° C.) 200° C. lower than the melting point (Tm) of the metal to produce a highly crystalline metal powder.
However, because nickel chloride is normally used as the nickel compound in the vapor phase chemical reduction method because of its high vapor pressure, the resulting metal nickel powder contains residual chlorine. The chlorine needs to be removed by washing because it can adversely affect the properties of electronic components, but washing is likely to cause aggregation, and separation may require long periods of time or complex processes. Moreover, the composition cannot be accurately controlled when preparing an alloy of metals with different vapor pressures.
With the spray pyrolysis method, on the other hand, highly crystalline or single-crystal metal powders and alloy powders which have a high purity, a high density and a high dispersibility can be obtained. However, because this method uses large quantities of solvent the energy loss during thermal decomposition is extremely high, and aggregation and splitting of the droplets also cause the resulting powder to have a broad particle size distribution, making it difficult to set the reaction conditions such as droplet size, spray rate, droplet concentration in the carrier gas and retention time in the reaction vessel so as to obtain a powder with a uniform particle size, and leading to increased costs because the dispersion concentration of the droplets cannot be increased. Because evaporation of the solvent occurs from the surfaces of the droplets, moreover, they are likely to become hollow or split when the heating temperature is low.
In comparison with the spray pyrolysis method, the method of thermally decomposing a solid metal compound powder in a gas phase offers the advantages, for example, of no energy loss due to evaporation of the solvent, high efficiency because the raw material powder is not prone to aggregation and splitting and can be dispersed at a relatively high concentration in the gas phase, and the fact that a solid powder with good crystallinity can be obtained even at a relatively low temperatures. However, further increasing the dispersibility requires more energy or special dispersion equipment to increase the ejection speed into the reaction vessel for example, and the raw material powder must be even finer when manufacturing an extremely fine metal powder, making particle size adjustment and dispersion difficult. Moreover, when cheap, easily available cost nickel nitrate powder or nickel nitrate hydrate powder is used as the raw material, because these compounds are extremely hygroscopic the particles tend to stick together, and also tend to adhere to and block the disperser and nozzle, making the powder itself difficult to deliver to the reaction vessel in a dispersed state.